How does ADO.NET connection pooling work, and how would you diagnose a connection-pool timeout in a .NET API?

ADO.NET connection pooling and pool exhaustion

Explains physical connection reuse, pool partitioning, short ownership, exhaustion causes, diagnostics, and why increasing the pool is not the first fix.

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Opening a SqlConnection normally borrows a physical connection from a client-side pool, and closing or disposing it returns that connection for reuse. Pools are separated by exact connection configuration and other context such as identity, so small connection-string differences can fragment them. If the maximum pool size is reached and no connection returns before the connection timeout, the request fails. I open as late as practical, dispose deterministically with using or await using, and do not keep connections or readers across unrelated work.

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ADO.NET, SqlConnection, connection pooling, exact connection strings, disposal, max pool size, open readers, transactions, concurrency, timeouts, and diagnostics.

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Opening a database connection is expensive, so ADO.NET reuses physical connections. During incidents, a pool timeout often means application work is holding scarce connections too long rather than that the database rejected a login.
Opening a SqlConnection normally borrows a physical connection from a client-side pool, and closing or disposing it returns that connection for reuse. Pools are separated by exact connection configuration and other context such as identity, so small connection-string differences can fragment them. If the maximum pool size is reached and no connection returns before the connection timeout, the request fails. I open as late as practical, dispose deterministically with using or await using, and do not keep connections or readers across unrelated work.
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Assuming Close always closes the physical database connection.
Increasing max pool size before finding why connections are held.
Keeping a reader open while doing slow network or CPU work.
Creating many nearly identical connection strings and therefore many pools.

How does ADO.NET connection pooling work, and how would you diagnose a connection-pool timeout in a .NET API?

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